This is a toughie, but one you can be sure you'll be asked. Definitely keep things positive—you have nothing to gain by being negative about your past employers. Instead, frame things in a way that shows that you're eager to take on new opportunities and that the role you’re interviewing for is a better fit for you than your current or last position. For example, “I’d really love to be part of product development from beginning to end, and I know I’d have that opportunity here.” And if you were let go? Keep it simple: “Unfortunately, I was let go,” is a totally OK answer.
What your interviewer is really trying to do with this question—beyond identifying any major red flags—is to gauge your self-awareness and honesty. So, “I can't meet a deadline to save my life” is not an option—but neither is “Nothing! I'm perfect!” Strike a balance by thinking of something that you struggle with but that you’re working to improve. For example, maybe you’ve never been strong at public speaking, but you've recently volunteered to run meetings to help you be more comfortable when addressing a crowd.